Fuel filling stations may provide fuel that varies by cetane number. The cetane number of a fuel provides a reference to the ignition delay of the fuel. In particular, higher cetane fuels have a shorter ignition delay and lower cetane fuels have longer ignition delay. The only fuel cetane requirement in the U.S. is for cetane to be greater than 40, and testing has revealed that consumer diesel fuel cetane may vary at least from 40.3 to 56.9 from time-to-time. Although it may not be useful to determine a specific cetane number for a combusted fuel during engine operation, it has been recognized that engine emissions can vary with cetane because the variation in ignition delay can affect combustion products of a cylinder air-fuel mixture. Therefore, it may be desirable to recognize fuels that have different cetane numbers.
In International Patent Application Publication WO2009063298 the inventors describe a method for estimating the cetane number of a fuel based on estimated injection timing. The method appears to determine a fuel cetane number from injection timing at which misfiring starts to occur, the misfire is based on a variation in the increase in engine torque as the timing of the fuel injections are shifted. After the cetane number is determined, the method appears to decrease fuel injection pressure as start of injection (SOI) timing is retarded. However, the method may not be suitable for low temperature combustion where longer ignition delay may result in formation of lower amounts of combustion particulate matter and where shorter ignition delay may result in formation of increased amounts of combustion particulate matter.
The inventors herein have recognized the above-mentioned disadvantages and have developed an engine operating method, comprising: combusting an air-fuel mixture in an engine where a delay from end of fuel injection to start of combustion is present; and adjusting fuel injection pressure and start of fuel injection timing in response to feedback of the air-fuel mixture combustion, the fuel injection pressure increasing as start of fuel injection timing is retarded.
By increasing fuel injection pressure and retarding start of fuel injection timing, it may be possible to provide combustion heat release during low temperature combustion for higher cetane fuel that is similar to that of a nominal cetane fuel. Further, fuel injection pressure can be reduced and start of fuel injection timing can be advanced so that combustion heat released for a lower cetane fuel is similar to that of a nominal cetane fuel. In this way, the injection timing and pressure adjustments for a fuel having a cetane number that varies from a nominal cetane number can be adjusted so that engine emissions are similar even when a different level cetane fuel is combusted.
The present description may provide several advantages. For example, the approach may reduce engine emissions when fuels with varying cetane numbers are combusted in an engine. Further, the approach may be included as part of a system to control torque from combustion feedback. Further still, the approach may improve vehicle drivability during some conditions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.